1. Field of the Invention
The present invention relates to a combination of a machine tool, a numerical controller for controlling the machine tool, and an on-machine measuring device mounted on the machine tool and configured to make a measurement on the machine tool.
2. Description of the Related Art
For high-precision machining, it is very important to measure the three-dimensional shape of a workpiece on a machine tool. Conventionally, a probe-type shape measurement sensor is known to be used to measure the shape of a workpiece in such a manner that the tip of a shape probe is brought into contact with the workpiece as the three-dimensional shape of the workpiece is measured.
Japanese Patent Application Laid-Open No. 2006-21277 discloses an example of a technique in which a workpiece is measured on a machine tool that carries a shape measurer (on-machine measuring device) thereon. According to the technique disclosed in this patent document, a numerical controller simultaneously controls three axes, X-, Y- and Z-axes, of the machine tool and outputs position commands individually to servomotors for the respective axes. A probe of this on-machine measuring device includes a contact that contacts a surface of the workpiece and a laser interferometer that converts a fine displacement of the contact into a laser signal. The numerical controller is connected to a personal computer by an optical cable. The output signal of the probe that is indicative of displacement of the contact is delivered to the personal computer, in which necessary arithmetic operations for the measurement are performed. The personal computer captures position information from the numerical controller on a real-time basis.
According to the prior art technique, the signal from the on-machine measuring device is delivered to the numerical controller, and signals from the moving axes of the machine tool to the numerical controller. In this case, however, it is difficult to fetch the signal from the measuring device in synchronism with the signals from the moving axes of the machine tool into the personal computer.
In order to synchronize a signal ip from an on-machine measuring device 1 with signals ipx, ipy and ipz from position detectors for moving axes (X-, Y- and Z-axes 3, 4 and 5) of a machine tool, as shown in FIG. 9, moreover, the signals from the position detectors must be branched by means of independent signal branching devices 13 and input to a personal computer 11. These signal branching devices 13 for branching the signals from the position detectors have to be mounted as many as the moving axes required for on-machine measurement. In FIG. 9, position detectors from B- and C-axes 6 and 7, out of the moving axes of the machine tool, and branching devices for these signals are omitted. In FIG. 9, moreover, reference numerals 8 and 10 denote a numerical controller and a base of the machine tool, respectively.
However, the signal branching devices for branching the signals from the position detectors are very expensive. Also, the signal branching requires dedicated position detector substrates and more cables for them, so that complicated wire arrangement is essential. In addition, a power source is needed to supply electric power to the signal branching devices.
In order to receive the signals from the position detectors for the moving axes that are branched by the signal branching devices, moreover, the personal computer must be fitted with a dedicated signal processing board. Therefore, the computer should be provided with a mother board that has a sufficient number of slots for mounting the signal processing board so as to be congenial with the processing board.
Further, the signals from the position detectors for the moving axes of the machine tool involve pitch errors that are attributable to mounting errors and the like of the position detectors. Normally, the errors are minimized by performing pitch error correction with a numerical controller. Since the branched signals obtained by branching the signals from the position detectors with the signal branching devices are raw signals with their pitch errors uncorrected, however, data stored in the personal computer must be subjected to separate pitch error correction.
Thus, on-machine measurement with on-machine measuring device attached to a machine tool requires complicated devices and high costs, and processing the information obtained by such a measuring device entails extended time.
Corrective machining based on on-machine measurement is indispensable for machining with a high shape accuracy of nanometer order in ultra-precision machining. Therefore, a measurement signal from the on-machine measuring device and position detection signals from the position detectors for detecting the positions of the moving axes of the machine tool must be stored in a storage device of an external device, such as a personal computer, in such a manner that they are completely synchronized (i.e., as data based on the same measurement timing). In order to improve the reliability of the measurement and reduce costs, moreover, interfaces of the on-machine measuring device should be made as simple as possible.